Process of roasting chromite ore to produce sodium chromate



s J. HULTMAN ETAL PROCESS OF ROASTING CHROMITE ORE Aug. 15, 1967 3,336,102

TO PRODUCE SODIUM CHROMATE Origindl Filed July 12, 1963 FIG.

0 0 001mm 5 r0 mow J4 rrok/vn United States Patent Ofiiice 3,336,102 PROCESS OF ROASTING CHROMITE ORE TO PRODUCE SODIUM CHROMATE Stanley J. Hultman, George L. Rich, Donald B. Bailey,

and Donald E. Tunison, Jr., Corpus Christi, Tex., assignors to Pittsburgh Plate Glass Company, Pittsburgh, Pa., a corporation of Pennsylvania Continuation. of application Ser. No. 294,601, July 12, 1963. This application Dec. 16, 1966, Ser. No. 602,425 4 Claims. (CI. 2356) This application is a continuation of application Ser. No. 294,601, filed July 12, 1963, and 'now abandoned.

This invention relates to the roasting of chromium ores. Such ores contain chromium in the form of trivalent chromium oxide bound together either chemically or physically with other metals in the oxide form, such as iron, aluminum, vanadium, silicon, and the like. It has long been known to roast chromium o'res of this type with sodium carbonate, with or without the presence of lime, in order to render the chromium values water soluble. In the course of this roasting, the chromium is transformed from the trivalent state to the hexavalent state. As such, a good share of the chromium may be extracted from the resulting roast by simple extraction with water.

In theUnited States, the roasting operations of this type have conventionally been conducted in long rotary kilns which are essentially heated cylinders in which the mixture to be roasted is fed in at one end and the roasted material exits from the other end while the cylinder rotates. This process, while successful commercially, is open to certain objections.

In the first place, it is diflicult to seal from the surrounding atmosphere. Consequently, dusts which are formed in the kiln as the roast tumbles therein frequently escape in gaseous suspension from the kiln. Since chromium in hexavalent form has a degree of toxicity, the atmosphere adjacent an operating rotary kiln, can produce a health hazard.

Because of the intrinsic objections to a rotary kiln and because horizontal hearths are more easy to seal, it has been proposed to make use of such hearths in order to roast the chromium ore-alkali mixtures. Apparently, this process has achieved some success in Europe, although it has not heretofore been successfully applied in the United States.

In the investigation of the horizontal hearth process resulting in the present invention, a number of ditficulties have been observed. In the first place, serious difliculty has been encountered in obtaining complete or substantially complete solubility of the chromium. Furthermore, the roasting operation, particularly the oxidation of the chromium from trivalent to hexavalent state, was found to take place at an unduly slow rate. This resulted in low outputs of the furnace. In addition, an unduly high heat consumption has been observed, as well as substantial dust losses out the stack.

According to the present invention, methods have been developed for improving substantially the operation of a rotary hearth furnace in the roasting of these chromium ore mixtures. Thus, by the practice of the invention herein contemplated, it has been possible to effectively roast the ore, achieving a roast in which at least about 85 percent of the chromium content thereof is soluble in water and 90 to 95 percent or more of the chromium content is soluble in an acid, such as sulphuric acid. In addition, it has been possible to increase the rate of roasting and oxidation to hexavalent state by as much as 25 to 50 percent, and to substantially improve heat efficiency. Furthermore, it has been found that this operation can be conducted with less diluent, or even no diluent, than has been characterized by previous roasting operations.

3,336,102 Patented Aug. 15, 1967 In the rotary kiln process, it has been common practice to recycle large amounts of previously extracted mud. This mud, which serves largely as a diluent but also as a source of chromium, has been necessary since in the rotary kiln process it has not been possible to permit production of a roast containing more than about 28 percent by weight of acid soluble chromium. When this value is exceeded, the roast becomes quite sticky, agglomerates badly, and is difficult to handle or process. In contrast, in the practice of applicants process, it is possible to produce a roast containing as much as 35 percent by weight of acid soluble chromium without encountering any serious difficulties in subsequent processing. This, again, improves the efficiency and through-put of the operation.

In accordance with this invention, a mixture of chromite ore, a sodium carbonate, and lime is deposited in the form of a layer upon a rotating horizontal hearth in a furnace. The hearth is heated by directing burning gas over the layer, usually in a horizontal direction and transverse to the direction of rotation of the hearth.

In order to ensure oxidation of the chromium from trivalent to hexavalent state, air is fed into the space above the layer of ore undergoing roasting. However, to achieve the results discussed above, the manner in which the air is introduced is of considerable importance.

Thus, the air, or at least a major portion of the air supplied for the oxidation, should be fed into the space above the layer of ore but below the burning gas heating the furnace. By this means, an effective oxidation of the chromium values in the ore is obtained. As a general rule, there is established a substantial concentration of oxygen (rarely below 5 percent and usually ranging between 5 to 12 percent by volume) in the gas immediately above the ore undergoing roasting. This concentration is established notwithstanding the high amount of carbon dioxide, diluent and nitrogen introduced into the furnace by virtue of the burning gas-air mixture used to heat the furnace.

In accordance with a further embodiment of the invention, a mixture of the chromite ore, lime, soda ash, and sodium bicarbonate is fed into the furnace. The sodium bicarbonate apparently imparts a beneficial elfect in that it also ensures production of a readily processable roast which contains high percentages (above percent) of acid soluble chromium, based upon the total chromium therein, rapidly and Without recourse to recycling waste mud or at least substantial quantities of waste mud. The exact reason for this is not understood. Quite possibly it may be due to the fact that sodium bicarbonate decomposes, liberating carbon dioxide, at a temperature some what lower than the temperature at which the roasting is conducted.

It will be understood that a layer of material undergoing roasting by heat applied from above, as herein contemplated, will have a temperature gradient, the hottest portion of the roast being at the top of the layer, the cooler portion being at the bottom resting on the rotating table. Quite possibly the sodium bicarbonate, as it decomposes, supplies a considerable amount of gas which passes through the upper hotter portion ofthe bed over a considerable area of the roast, keeping it porous and ultimately facilitating diffusion of oxygen into the bed. Whatever the reason, the results of the operation are effective and advantageous.

The manner in which the process is conducted will now be described in .greater detail by referring to the accompanying drawings, in which:

FIG. 1 is a vertical view, partially in section, diagrammaticallyillustrating the structure of the furnace used in the performance of this process; and

FIG. 2 is a diagrammatic plan view illustrating in a general way the various stations for feeding ore, withdrawing roast, and feeding air and burning gas.

As shown in FIG. 1, a furnace 10 is provided in which there is a ring or doughnut-shaped heating chamber 11. A ring-shaped table 12 is supported in the lower portion of the chamber 11, resting on trunnions 32 which roll on floor 34 or on rails (not shown) disposed in such floor. The chamber is enclosed by refractory walls 14 and 18 and refractory roof 30. A feed screw 16 is provided at one station slightly above the table. The furnace is heated by a plurality of circumferentially spaced burners 22 which project burning gas into and across the chamber 11 and well above, for example, 18 to 30 inches above the table.

Oxygen ports 20 also are circumferentially spaced about the peripheral walls of the chamber for introduction of air or oxygen into the chamber. These ports are disposed below the burners but above the table a distance high enough so that the projected air is directed at a point from 6 to 18 inches above the layer of roast on the table. As illustrated, both the burning gases projected from burners 22 and the air projected from ports 20 flow in a direction transverse to the direction of movement of the table and across the chamber. Although, as illustrated in FIG. 1, the burners and ports are shown on the outside wall, it is also possible to have the burners and ports on the inside wall 14 or on both inside and outside walls (see FIG. 2).

FIG. 2 diagrammatically illustrates the location of the various burners and oxygen ports and the various stations. Thus, there is provided a feed chute 42 for feeding the mixture to be roasted located on the outer edge or periphery of the hearth. Also, the discharge screw is spaced circumferentially a distance, for example 30 to 45 degrees around the table, from the feed chute and feeds into a discharge chute 44 which is located adjacent the inner edge of the rotating table 12. Pluralities of burners 22 are shown spaced circumferentially along both the inside and outside walls; those inside being directed across the chamber toward the outer wall and those intersecting the outer wall being directed across the chamber toward the inner wall. Likewise, the air ports 20 are circumferentially spaced around both the inside and outside walls and, like the burners, those intersecting the inside wall are directed toward the outer wall and vice versa. As shown in FIG. 1, the burners and the air ports may, if desired, be located in but a single wall. At another station, as illustrated, the furnace flue 36 is disposed.

In the operation of the device, the ore-carbonate of soda-lime mixture is fed through chute 42 and deposited upon the rotating table 12. In the course of further rotation of the table, the deposited mixture reaches the discharge screw and is rabbled and fed inwardly. Consequently, the roasted material tends to spiral inwardly along the table, finally reaching the inner edge of the table, and is discharged by the rotating screw into chute 44. Enough fuel is burned in the chamber 11 to cause the temperature of the roast to approximate about 1900 to 2000 F. While some deviation from this range is possible, it is rare that the temperature of the roast being discharged falls below about 1800 F. or exceeds 2200 F.

The rate of feed normally is adjusted to establish a bed which is about 3 to 6 inches in depth. The speed of rotation and the rate of feed of mix and discharge of roasted mix is adjusted so that the discharge product will have at least about 85 percent of its chromium content in water soluble state. Oxygen in the form of air, which may also be oxygen-enriched air, is fed into ports 20. The rate of oxygen feed is normally high enough to hold the oxygen content of the gas just above the layer at least percent by volume throughout the major period, preferably 75 percent or more of the roasting period. Usually, the oxygen content does not exceed more than about 12 percent by volume at this level when ordinary air is fed through the ports. By feeding pure oxygen or oxygenenriched air, an oxygen concentration above to 12 percent by volume may be established and rate of roasting appreciably increased.

The combustion gases, which contain 5 to 15 percent by volume of carbon dioxide together with large volumes of nitrogen, escape from the chamber space through the stack 36.

As a general rule, the mix which is fed to the furnace is a mixture of chromite ore, a sodium carbonate and lime (including dolomitic lime) containing a high CaO concentration. The amount of sodium carbonate (whether in the form of sodium carbonate or sodium bicarbonate) should be sufficient to react with the major portion of the chromite ore to form sodium chromate. While some deficiency of sodium carbonate is permissible, the amount is rarely less than percent of that required stoichiometrically to combine with the chromium to produce sodium chromate, and often there is an excess of the stoichiometric, rarely more than 25 percent excess. Usually, the amount of sodium carbonate ranges from 0.5 to 0.8 pound per pound of chromite ore.

In order to hinder solution of aluminum and vanadium components in the ore upon subsequent leaching of the roasted mix, it is advantageous to have substantial amounts of calcined lime (CaO) in the mix. This amount normally exceeds about 15 percent by weight of the mixture, but rarely is in excess of about 35 to 40 percent by weight. On the basis of the ore, a good range is about 0.7 to 1.1 pounds per pound of ore.

Whereas most of the mixtures previously used in the prior art also make use of recycled or previously processed ore residue, commonly known as waste mud, the use of this mud may be dispensed with since, in the present process, it is possible to extract substantially all of the chromium values from the roast produced, and recourse to a diluent is not necessary. Of course, some concentration of recycled mud may be added if desired. However, this merely reduces the capacity of the furnace and thus the amount of mud introduced rarely exceeds more than about 0.3 pound per pound of ore being fed, and usually is omitted entirely.

The mix is prepared in convenient manner by feeding the various ingredients to a mixing apparatus, such as the interior of a rotating cylindrical tube, where the products are tumbled together until an essentially uniform product is obtained.

As is well known in the art, the ultimate roast is leached with water to recover the chromium values in the form of sodium chromate. This sodium chromate is a relatively concentrated solution containing in excess of 20 percent by weight of sodium chromate, the amount often being as much as 40 to 45 percent by weight of this material. For many purposes, sodium dichromate is required. Sodium dichromate can be prepared by reaction of sodium chromate with carbon dioxide. In the practice of this process, as described in US. Patent No. 2,931,704, the carbon dioxide is reacted with sodium chromate, with consequent production of sodium bicarbonate which is separated from the resulting sodium dichromate aqueous solution.

While all of the sodium carbonate used may be Na CO it has been found that the sodium bicarbonate thus obtained can be very effectively used as a source of sodium carbonate in the roasting operation. Thus, 25 to percent of the carbonate of sodium in the mix fed to the furnace may be in the form of sodium bicarbonate, the balance thereof being sodium carbonate. Often, however, at least 25 percent of the alkali metal carbonate in the mix to be roasted is sodium carbonate Na CO In the mixing of the various ingredients, as described above, some heat is evolved. This is due partly to the presence of some water in the product which slakes calcined lime in the mixture, thus evolving heat. Also, some amount of water may be added in order to ensure heating the mixture, although the amount of water should 5 be held low enough to ensure production of a dry pulverulent mixture. A further reaction can take place between lime and part of the sodium bicarbonate. The temperature of the mixture ranges from about 200 to 300 F., rarely above 500 F. and usually 230 to 250 F. As a consequence, the mixture is a very dry, fluify, freeflowing material, the flow of which can be readily controlled at a uniform rate onto the furnace table.

By practicing in the above described manner, a well roasted product containing in excess of 30 percent of acid soluble chromate, expressed as Na C-rO usually in the range of 35 percent acid soluble chromate, in which at least 85 percent of the chromium is water soluble and at least 90 percent of the chromium is soluble in sulphuric acid, is obtained. This product can be readily leached with Water to produce a concentrated sodium chromate solution which contains but negligible quantities of impurities, such as vanadium or aluminum. Rarely does the concentration of either of these materials exceed 0.5 percent by weight, and usually they are below 0.01 percent by weight, based upon the sodium chromate in the solution.

Substantially all of the chromium values of the roast can be obtained in a simple two-stage extraction Where, in the first stage the roast is extracted with water, and in the second, the solid residue from the first extraction is extracted with water in the presence of sodium carbonate and carbon dioxide at a pH of 8 to 10, preferably 8.5 to 9.5.

Chromite ores which are roasted according to this invention normally contain at least 40 to 45 percent by weight of chromium vcalculated as Cr O A typical ore which can be effectively treated has the following analysis calculated as the oxides of the respective metals:

Percent by weight Cr O 44.8 FeO 25.1 A1 15.3 CaO 0.2 MgO 11.5 SiO 2.5 V 0 0.3

The design of the air ports may be varied widely. Often they comprise a plurality (3 or more) of small ports disposed immediately below a burner. To avoid unduly cooling the roast, this air or oxygen may be preheated above 500 F., for example 700 to 1000 F., before introducing it into the roasting chamber.

While calcined limestone has been described as the source of lime, calcined dolomitic limes which contain 50 mol percent of their alkaline earth metal content as calcium also may be used.

While the oxygen ports illustrated in FIG. 1 are shown to be disposed below the burners 22, other embodiments are possible so long as a major part of air ultimately is directed into the level below the burners. Thus, the air can be introduced above the burners, or at the level thereof, and deflected or directed downwardly to establish the desired oxygen concentration.

Although the present invention has been described with reference to specific details of certain embodiments, it is not intended that such details shall be regarded as limitations upon the scope of the invention except insofar as such limitations are included in the accompanying claims.

We claim:

1. The process of roasting chromite ore with a member of the group consisting of sodium carbonate, sodium bicarbonate and mixtures thereof to render the chromium values of said ore water soluble and produce sodium chromate which comprises depositing a layer of a mixture of ore and the said member on a horizontal rotating hearth in a furnace, directing burning gas into a space in the furnace above the said layer and feeding air into said space to oxidize chromium of said ore to hexavalent state thereby rendering it Water soluble and producing sodium chromate, the major portion of said air being fed below the burning gas but above the said layer, said air supplying in the gas immediately above the layer of ore a concentration of oxygen approximately 5 percent by volume.

2. A process of roasting chromite ore with a member of the group consisting of sodium carbonate, sodium bicarbonate and mixtures thereof to render the chromium values of said ore water soluble and produce sodium chromate which comprises depositing a layer of a mixture of ore and the said member on a horizontal rotating hearth in a furnace, introducing heat into the space above the layer to roast the ore while introducing oxygen to said space and maintaining the air content of the atmosphere immediately above the layer and below said heat introduction above 5 percent by volume, thereby oxidizing chromium of said ore to render it water soluble and produce sodium chromate.

3. The process of roasting chromite ore with lime and a member of the group consisting of sodium carbonate, sodium bicarbonate and mixtures thereof to render the chromium values of said ore water soluble and produce sodium chromate which comprises depositing a layer of a mixture of ore, lime and the said member on a horizontal rotating hearth in a furnace, directing burning gas into a space in the furnace above the said layer and feeding air into said space to oxidize chromium of said ore to hexavalent state thereby rendering it Water soluble and producing sodium chromate, the major portion of said air being fed below the burning gas but above the said layer, said air supplying in the gas immediately above the layer of ore a concentration of oxygen approximately 5 percent by volume.

4. A process of roasting chromite ore with lime and a member of the group consisting of sodium carbonate, sodium bicarbonate and mixtures thereof to render the chromium values of said ore water soluble and produce sodium chromate which comprises depositing a layer of a mixture of ore, lime and the said member on a horizontal rotating hearth in a furnace, introducing heat into the space above the layer to roast the ore while introducing air into said space and maintaining the oxygen content of the atmosphere immediately above the layer and below said heat introduction above 5 percent by volume, thereby oxidizing chromium of said ore to render it water soluble and produce sodium chromate.

No references cited.

OSCAR R. VERTIZ, Primary Examiner. B. H. LEVENSON, Assistant Examiner. 

2. A PROCESS OF ROASTING CHROMITE ORE WITH A MEMBER OF THE GROUP CONSISTING OF SODIUM CARBONATE, SODIUM BICARBONATE AND MIXTURES THEREOF TO RENDER THE CHROMIUM VALUES OF SAID ORE WATER SOLUBLE AND PRODUCE SODIUM CHROMATE WHICH COMPRISES DEPOSITING A LAYER OF A MIXTURE OF ORE AND THE SAID MEMBER ON A HORIZONTAL ROTATING HEARTH IN A FURNACE, INTRODUCING HEAT INTO THE SPACE ABOVE THE LAYER TO ROAST THE ORE WHILE INTRODUCING OXYGEN TO SAID SPACE AND MAINTAINING THE AIR CONTENT OF THE ATMOSPHERE IMMEDIATELY ABOVE THE LAYER AND BELOW SAID HEAT INTRODUCTION ABOVE 5 PERCENT BY VOLUME, THEREBY OXIDIZING CHROMIUM OF SAID ORE TO RENDER IT WATER SOLUBLE AND PRODUCE SODIUM CHROMATE. 